Control all transition alpha with TransitionViewModels

Part of the keyguard-shade migration process relied on
KeyguardInteractor.keyguardAlpha, which received data from legacy
sources such as NotificationPanelViewController. Now that all
transitions and view models are in place, deprecate this legacy source
for fined-grain transition control between KeyguardState.

This also simplies the flows into a straight merge.

Fixes: 322198222
Fixes: 324678582
Test: atest com.android.systemui.keyguard
Flag: ACONFIG com.android.systemui.keyguard_shade_migration_nssl
DEVELOPMENT
Flag: ACONFIG com.android.systemui.migrate_clocks_to_blueprint
DEVELOPMENT
Flag: ACONFIG com.android.systemui.keyguard_bottom_area_refactor
STAGING

Change-Id: Ib708ac689b5feff32559a812939d808c19de0246